I ran into this problem once before, asked around the office and my professor from my Timber Design class. They all told me to avoid it. I'd be interested to hear if anybody has any good suggestions on how.
I suspect the torsional strength is too low and unreliable for timber torsion design to be worthwhile.
Timber's natural tendency to split along the grain will be exacerbated by torsion.
The aitc timber designers handbook has some info on this.
I dont have a copy, but I recall that it basically follows classic torsion theory for a solid rectangle and compares the shear and acial stress values to the allowable.
